Yes, low level of harmonics generated by intermodulation, but 5 microseconds of phase intermodulation is already audible as something artificial. Especially on stereo, when it means already uncertain stereo image localization. It sounds like paradox, but much easier measurable effects caused by Doppler effect in speakers is less audible! My understanding is, because human perception got used to mechanical alterations during long time of evolution, while electrical errors are something new in the recent life of each individual.

On my learning front that was initiated by the question of this thread, I have three questions for today:
Conventional input stage. LTP etc. The major value of feeding the legs with a current mirror is that when using only one output, it is hard to predict/manage the impedance of the second stage. Forcing balance with less than perfect diff transistor is secondary. The current mirror forces balance in the diff pair regardless. Am I starting to get that correct? Second, I have not seen the Wilson current mirror in any of the schematics I have. Is it past the diminishing returns point? Thirdly, if I power the input stage from a regulated supply, is there as much advantage to have a CCS in the tail? I can read the books, but they don't answer questions yet. It seems to me I have seen comments something like "the CCS measures great, but does not sound the best" which does not make complete sense but I am sure there are a hundred smaller factors of which I know not.

If I put all of these questions together, it leaves a fourth bonus question: If one uses a matched pair i.e single die, for the current mirror, is the diff pair as critical? This would leave open the possibilities for JFETS not available as monolithic pairs.

This is where subjectivists and objectivists meet. I understand Fremer does his listening evaluation before Atkinson does his measurements, so thumbs up for his ears.

If anything, it also shows that further reduction of measurable deviations from perfect, may still pay out in acoustical terms.

The interesting thing though is that other amps that measure badly can still sound great. I don't know how to link to that article in Stereophile about the WAVAC, but it doesn't do well on the test bench, that is one thing I remember. However, the listening tests were very positive, and there sonic quality is substantiated by other samplers. I remember a recent post by John Curl mentioning WAVAC as one of the highlights of the show.

So, there are two possibilities here. Either the WAVAC is a perfect effects box. Or, amps like the WAVAC and the Soulution do something else right, something which still evades measurement, but which is essential in producing the most natural sound without audible artifacts.

What are we really looking for here as depicted in the below screen shots, what is good better , best !!!!

I would say the first picture is the best because it show the largest randomness, i.e. noise, versus a repetitive distortion residue. Secondly, the repetitive part seems to be mainly 3rd order distortion if I counted right, not that much of a problem. Interesting is that the repetitive part of the residue is neatly phase aligned with the fundamental. Could that be a clue to something?

If anything, it also shows that further reduction of measurable deviations from perfect, may still pay out in acoustical terms.

Of course it does!
For example, line array of small speakers from floor to ceiling sounds much more natural than traditional speakers with single drivers. For the same sound pressure drivers in line arrays do much shorted excursion, that greatly reduces distortions generated by them. And so on. The point is, efforts have to be applied where they lead to audible results, instead of lowering down THD of power amp from 0.01 to 0.001% where such reduction is inaudible, but means to reduce THD cause another audible artifacts.

I would say the first picture is the best because it show the largest randomness, i.e. noise, versus a repetitive distortion residue. Secondly, the repetitive part seems to be mainly 3rd order distortion if I counted right, not that much of a problem. Interesting is that the repetitive part of the residue is neatly phase aligned with the fundamental. Could that be a clue to something?

vac

With the third one being the worst ...? OK so better to have the random noise than for it to follow the signal as in pic 3 ...?

"So, there are two possibilities here. Either the WAVAC is a perfect effects box. Or, amps like the WAVAC and the Soulution do something else right, something which still evades measurement, but which is essential in producing the most natural sound without audible artifacts."

I vote for number two. That of course does not preclude amps that measure very well from also sounding very pleasing.